At the present time, loadings to an unequipped shoreline are carried out using bay crafts, landing ships or air cushion crafts.
A bay craft or a landing ship is characterized with a shallow draft allowing landings on shoreline of gentle gradient, but has the drawback of low speed due to its hull shape and limited capability for movement and seaworthiness as a result of its shallow draft and the fact that it is rather top heavy.
An high see ship, passengers or cargo vessel, allowing fast transit, good maneuverability and seaworthiness is characterized with a deep draft which prevents it from getting close to a gentle sloping shore.
Convertible vessels had been considered to change a deep-sea catamaran into a shallow water craft. This type of convertible vessels has been described by Malin (U.S. Pat. No. 3,437,067), Zadrowny (U.S. Pat. No. 3,898,946), Matsumoto (JP 5,921,195), Yilmaz (U.S. Pat. No. 5,921,195). The conversion was considered through a movable buoyant body of the vessel raised or lowered between the hulls, the movable body (or platform) being connected to adjacent hulls.
Some improvements are proposed in the present invention to solve various problems occurred in the definition of these convertible vessels.
First, one must be able to perform the conversion in heavy seas. The conversion is needed in open seas near the coast between shore (platform in lower position) and deep-sea (platform in upper position). The conversion is needed when loading/unloading in deep sea on a logistic vessel, the ro-ro transit being only possible when the platform is in the lower position. The conversion is also needed in deep seas when entering/leaving the well dock of a landing ship, the platform being in an intermediate position to avoid the suctions effect of the very shallow waters of the well and avoid a too great height not compatible with well dock clearances. This suction effect is observed on all existing landing ships entering/leaving a narrow well of a dock landing ship and is the origin of many bottom damages.
A conversion have some similarities to cargo handling, Farrel (U.S. Pat. No. 3,537,413), Broes (U.S. Pat. Nos. 3,786,772, and 3,908,573), Hoehne (DE 30,19 706), and Kirby (U.S. Pat. No. 4,011,825) had detailed calm water cargo handling of buoyant cargos but not a conversion in heavy seas. In a conversion in heavy seas, the connections of the movable body of the vessel to adjacent hulls are subject to wedging according the large deformations of the hulls (torsion on quarter's seas, transversal flexion on beam seas, longitudinal flexion on head seas).
Second, the conversion of fast heavy cargos catamaran into a shallow water craft induces much body plan design difficulties to accommodate a propulsion system able to work either in deep-sea catamaran configuration and shallow water craft configuration. The body plan has to maximize the buoyancy in shallow waters. The body plan has to minimize the stern drag in catamaran configuration. Each of these requirements is antagonist with the other and the body plan becomes unrealistic without special design when the convertible vessel is a high speed and heavy loaded vessel.
Third, the housing of the connections of the movable body of the vessel inside the adjacent hulls needs a large space and cuts the whole shell plating. The structural hull strength is then affected. The hull hydrodynamic resistance is also strongly increased by such appendices or holes, thus reducing the vessel speed accordingly.
In the former patent EP 1,123,862, I have described such a convertible vessel, with a movable body not connected to adjacent hulls, independent of the catamaran structure, with a quick description of the moving means. The present patent describes precisions and improvements to face the structural and hydro-dynamical problems.